Photoflash lamp

ABSTRACT

A photoflash lamp having a thin film coating of highly adherent, low elongation polymeric resin on the exterior surface of its glass envelope, with a much thicker and higher elongation polymeric coating covering the thin film coating.

United States Patent [191 McDonough et al.

[ Aug. 27, 1974 PHOTOFLASH LAMP [75] Inventors: Thomas P. McDonough, Allenwood;

John W. Shaffer, Williamsport, both of Pa.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

[22] Filed: Nov. 20, 1972 [21] Appl. No.2 308,167

[52] US. Cl 431/94, 117/73, 117/94 [51] Int. Cl. F2lk 5/02 [58] Field of Search 431/94, 95; 117/72, 94, 1 17/73 [56] References Cited UNITED STATES PATENTS 2,791,113 5/1957 Anderson 67/31 3,258,356 6/1966 Caldwell et a1. 117/72 Primary Examiner-Carroll B. Dority, Jr. Assistant Examiner- Larry I. Schwartz Attorney, Agent, or Firm-Edward J. Coleman [57] ABSTRACT A photoflash lamp having a thin fihn coating of highly adherent, low elongation polymeric resin on the exterior. surface of its glass envelope, with a much thicker and higher elongation polymeric coating covering the thin film coating.

9 China, 2 Drawing Figures PHOTOFLASH LAMP BACKGROUND OF THE INVENTION This invention relates to photoflash lamps and, more particularly, to reinforcing coatings for the glass envelope of fiashlamps.

A typical photoflash lamp comprises an hermetically sealed glass envelope, a quantity of combustible material located in the envelope, such as shreddedzirconium or hafnium foil, and a combustion-supporting gas, such as oxygen. The lamp also includes an electrically or percussively activated primer for igniting the combustible in the flashlamp. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to coat the lamp envelope with a protective lacquer such as cellulose acetate.

The oxygen within the lamp is initially present at an elevated pressure, e.g., 8 atmospheres. During lamp flashing, the oxygen is heated, and the internal pressure rises to a peak value approximately 60 per cent higher than the initial value. Another event that takes place during flashing of the lamp is impingement of molten globules of metals and oxides from the actinic combustion onto the inner glass surface. Such thermal shock and stressing causes the glass to spall (or flake off small chips from the inner surface), crack, or grossly disintegrate. It is the function of the cellulose acetate lacquer coating to support the cracked or broken glass vessel and to generally maintain the integrity ,of the flashlamp during reaction.

The internal pressure, as stated previously, reaches a high peak value. This occurs early in the flash'cycle (e.g., l20 milliseconds). From that time on, the internal pressure within the lamp smoothly declines so that by a time of 160 milliseconds the internal pressure is approximately one atmosphere. It is seen, therefore, that the potential severity of lamp rupture isa time dependent function as it is determined by the difference between lamp internal pressure and atmospheric pressure. It is also apparent that the later in the flash cycle glass cracking or failure occurs, the better will be the containment characteristics of the lamp. It is for this reason that so-called hard glasses have been actively investigated by various flashlamp manufacturers.

Hard glasses, however, carry with them several disadvantages. They cost appreciably more per pound than does the traditional soft lead glass. Another fault of hard glasses is their very narrow thermal working range in comparison. to lead glass; that is, their viscosity changes very rapidly with melt temperature. This fact makes high speed automated manufacture of hard glass lamps more difficult, and again more expensive, than for soft lead glass.

SUMMARY OF THE INVENTION.

In view of the foregoing, a principal object of the invention is to provide a photoflash lamp having a stronger envelope structure for providing improved containment during flashing.

Another object of the invention is to provide a photoflash lamp having an envelope structure which is efiective to significantly delay cracking or failure of the glass thereof during flashing.

A further object is to provide an improved containment vessel for a flashlamp without introducing major changes in either lamp material costs or construction.

ture under conditions of lamp flashing. Whereas typical cellulose acetate lacquer coatings, as used commercially on tubular type flashlamps, have dried thicknesses of from about 0.005 to 0.015 inch, the coating described herein attains its advantages in thicknesses of 0.001 inch or less. In fact, increasing the thickness of the coatings described herein beyond about 0.001 inch or 0.0015 inch gives little further gain in performance. To continue to provide the necessary shatterresistance, however, a much thicker coating of a higher elongation material, such as cellulose acetate lacquer, is applied as an exterior coating over the firstmentioned low elongation coating.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully described hereinafter in conjunction with the accompanying drawings, in which:

FIG. 1 is an enlarged sectional elevation of an electrically ignitable photoflash lamp having reinforcing coatings in accordance with the invention;

FIG. .2 is an enlarged sectional elevation of a percussive-type photoflash lamp having reinforcing coatings in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The teachings of the present invention are applicable to either percussive or electrically ignited photoflash lampsof a wide variety of sizes and shapes. Accordingly, FIGS. '1 and2 respectively illustrate electrically ignited and percussive-type photoflash lamps embodying the principles of the invention.

Referring to FIG. 1, the electrically ignitable lamp comprises an hermetically sealed lamp envelope 2 of glass tubing having a press 4 defining one end thereof and an exhaust tip 6 defining the other end thereof. Supported by the press 4 is an ignition means comprising a pair of lead-in wires 8 and 10 extending through and sealed into the press. A filament l2 spans the inner ends of the lead-in wires, and beads of primer I4 and 16 are located on the inner ends of the lead-in wires 8 and 10, respectively, at their junction with the filament. Typically, the lamp envelope 2 has an internal diameter of less than one-half inch, and an internal volume of less than 1 cc., although the present invention is equally suitable for application to larger lamp sizes. A combustion-supporting gas, such as oxygen, and a filamentary combustible material 18, such'as shredded zirconium or hafnium foil, are disposed within the lamp envelope. Typically, the combustion-supporting gas fill is at a pressure exceeding one atmosphere, with the more recent subminature lamp types having oxygen fill pressures of up to several atmospheres.

As will be described in more detail hereinafter, the glass envelope 2 is reinforced, in accordance with the invention, by a first polymeric coating 19, which is applied in a very thin layer directly on the glass envelope and is selected to be highly adherent to'glass and have a percentage elongation of less than about 5 percent.

By highly adherent, we mean that the bond of the coating to the glass should be as strong as or stronger than the tensile strength of the coating 19 per se adhesiveness should equal or exceed cohesiveness. To complete the envelope structure, the coating 19 is covered by a much thicker coating 20 of polymeric material selected to have a percentage elongation of greater than about percent. We refer to percentage elongation as defined in ASTM Standard D638. Hence, the glass envelope 2 is reinforced with a relatively brittle, but highly adherent, first coating 19, which has been found to significantly increase burst strength, and a thicker and softer second coating 20, which provides shatter resistance.

The percussive-photoflash lamp illustrated in FIG. 2 comprises a length of glass tubing defining an hermetically sealed lamp envelope 22 constricted at one end to define an exhaust tip 24 and shaped to define a seal 26 about a primer 28 at the other end thereof. The primer 28 comprises a metal tube 30, a wire anvil 32, and a charge of fulminating material 34. A combustible 36, such as filamentary zirconium or hafnium, and a combustion-supporting gas, such as oxygen, are disposed within the lamp envelope, with the fill gas being at a pressure of greater than one atmosphere. In accordance with the invention, the exterior surfaceof the glass envelope 22 is covered with a thin, low elongation coating 45, which is similar to coating 19. Over the coating 45 is a thicker layer of high elongation coating 46, which is similar to coating 20.

The wire anvil 32 is centered within the tube 30 and is held in place by a circumferential indenture 38 of the tube 30 which loops over the head 40, or other suitable protuberance, at the lower extremity of the wire anvil. Additional means, such as lobes 42 on wire anvil 32 for example, may also be used in stabilizing the wire anvil, supporting it substantially coaxial within the primer tube 30 and insuring clearance between the fulminating material 34 and the inside wall of tube 30. A refractory bead 44 is fused to the wire anvil 32 just above the inner mouth of the primer tube 30 to eliminate tube 30 burn-through and function as a deflector to deflect and control the ejection of hot gases from the fulminating material in the primer. The lamp of FIG. 2 is also typian a submminutetypasnflgefirm'aimehstairs similar to those described with respect to FIG. 1.

Although the lamp of FIG. 1 is electrically ignited, usually from a battery source, and the lamp of FIG. 2 is percussion-ignitable, the lamps are similar in that in each the ignition means is attached to one end of the lamp envelope and disposed in operative relationship with respect to the filamentary combustible material. More specifically the igniter filament l2of the flash iir'fipiffrlo. 1 is incandesced electrically by current passing through the metal filament support leads 8 and 10, whereupon the incandescent filament 12 ignites the beads of primer l4 and 16 which in turn ignite the combustible 18 disposed within the lamp envelope. Operation of the percussive-type lamp of FIG. 2 is initiated by an impact onto tube 30 to cause deflagration of the fulminating material 34 up through the tube 30 to ignite the combustible 36 disposed within the lamp envelope. The invention is also applicable to other types of electrically ignitedlamps, such as those having spark gap or primer bridge ignition sturctures.

Many adherent, low elongation resins (i.e., those having a percentage elongation of less than about 5 P nt) have been evaluated for use as the thin reincommercial type M-3 lamps, and (2) crack time and rupture time tests using I-IP-ll lamps fabricated from 0.400 inch O.D. type 0010 glass tubing, having an internal volume of 0.78 cm, a charge of 30 mg of zirconium'shreds, and an oxygen pressure of 650 cm Hg absolute.

For purposes of ease of application by dipping, and attainment of the desired thin coating, the epoxy resin and curing agent were diluted with a volatile insert solvent such as, for example, toluene or methylene chlor ide. All data given represent coatings formed from a bifunctional bisphenol-epichlorohydrin epoxy resin having an expoxide equivalent weight of about 190, and a polyamine curing agent having an equivalent weight of 48. The weight ratio of resin to curing agent was 4.0 to 6.0. Coatings were cured 2 hours at 100 C.

I. Hydraulic Burst Test M-3 lamp vessels with the glass exhaust stem attached were filled with water and then attached to a hydraulic pump. A pressure transducer and electronic ;X-Y recorder were used to measure the internal hygdraulic pressure at the time of vessel failure. Note:

epoxy refers to a cured film.

Average Burst Test Vessel Strength (psig) Gain 1 Uncoated 474 l Coat epoxy (0.001" thick) 701 48 3 Coats epoxy (0.004" thick) 753 59 2,3,4 Uncoated 425 l Coat epoxy (0.001" thick) 628 60 5 Uncoated 42S Cellulose Acetate lacquer 528 24 (0.005 thick) dicating that the adhesion of the thin epoxy coating is greater than the internal strength of the epoxy resin.

11. Glass Crack Time Measurements The time of glass cracking was determined by a microphonic pickup of the sound accompanying such cracking, and an oscilloscope display. This technique was verified by use of high speed motion pictures. HP-l lamps were used in these tests.

This 50 to 90 percent gain in crack time is representative of many such tests. It should be pointed out that the wide variations in performance of the control groups are considered to be caused by variations insurface abrasion, etc., that the lamps have been subjected to.

111. Vessel Rupture Time In this test, HP-l lamps are placed in an evacuated chamber fitted with a pressure transducer and oscilloscope readout. The time at which the vessel ruptures is chamber pressure.

Average Rupture Time Test Vessel No. Lamps (milliseconds) Gain 1- Uncoated l0 56 0.00l" epoxy 135 137 2 Uncoated 8 111 0.001" epoxy l0 214 93 3 (high Uncoated 7 I04 pressure) 0.001 "epoxy 9 193 85 This test technique typically shows somewhat greater gains in the coated groups than does crack time measurement. This is probably because the earliest crack, which is the one recorded in crack time studies, does not always propagate or lead to vessel failure, Evaluation of highly adherent, bifunctional and polyfunctional epoxy resins of less than 5 percent elongation and different types of curing agents, such as amines and acid anhydrides, has shown no significant dependence of performance, as related herein, on epoxy resin type. Accordingly, with the exception of monofunctional epoxy resins and resins of more than 5 percent elongation or low adhesion, hundreds of different epoxy resins and hardener combinations could be used for coatingl9 or 45 with apparent equal utility and all fall within the spirit and intent of the inventive principles disclosed herein. Other thermosetting coatings such as for example, highly adherent acrylic, mela- 6 mine or urea formaldehyde systems o f less than 5 percent elongation also appear to be reasonably suitable for coating 19 or 45. Certain thermoplastic resins, such as nitrocellulose, have also been found to give some glass reinforcement, provided they are selected to be highly adherent to the glass and have a percentage elongation of less than 5 percent. In addition, the following materials can be used as the first coating 19 or 45, if the second, thicker coating 20 or 46 is selected 1 to be impervious to water vapor: zein, shellac, gelatin,

2 polyvinyl alcohol and egg albumin. By far, however, the

best results were obtained with an epoxy system. For

example, thermosetting acrylic enamels give at best a per cent increase in hydraulic burst strength over l5 indicated by an abrupt and permanent increase 1n the corresponding uncoated control.

The particular volatile solvent chosen and its relative quantity are not critical so long as it does not react with the other components present. The proportion of solvent used or, alternatively, the nonvolatiles content, is adjusted so' as to give the desired final coating thickness. For example, about 40 per cent by weight of either toluene or methylene chloride is suitable. A fter the initial gain in strength realized by the first 0.001 inch or so of coating 19 or 45, little further gain per unit coating thickness is available. Accordingly, we prefer to use a thickness of about 0.002 inch or less.

The outercoating 20 or 46 is applied in a much thicker layer and is comprised of a polymeric material having a higher percentage elongation, viz., greater than 5 percent elongation, such as cellulose acetate, polycarbonate, etc. For example, a suitable shatter resistant outer coating is provided by a layer of cellulose acetate lacquer having a dried thickness of 0.005 to 0.015 inch.

Although the, invention has'been described with respect to specific embodiments, it will be appreciated that modification s ahd changes maysefisae by those skilled in the art without departing from the true spirit and scope of the invention. In any event, however, the thin, inner coating 19 or must be highly adherent and have a percentage elongation of less than about 5 percent in' order to obtain the desired increase in burst strength. This is to be distinguished from the thin buffer coating for a flashlamp described in US. Pat. No. 2,791,113. This patent describes use of a buffer layer between the glass envelope and the cellulose acetate coating which is at least 0.002 to 0.003 inch thick and of such composition that it will melt and become ta'cky upon flashing of the lamp. The intent of the buffer coating is to insulate the much stronger outer coating of cellulose acetate lacquer from the hot glass vessel, and the tackiness is to prevent dispersion of glass particles should the bulb shatter. By virtue of the melting effect of the buffer coating it clearly would not increase vessel strength during flash. The materials suggested for the buffer coating are too soft and flexible to function in the manner we describe. We do not want our intermediate coating to melt on flashing; we want high initial adherence to the glass without melting.

On the other hand, a cured epoxy resin coating having a thickness of 0.001 inch or less is far too thin to provide any significant degree of thermal insulation for the overlying cellulose acetate coating. Further, the epoxy coating is thermosetting and does not melt on flashing.

Accordingly, the above-mentioned patent does not contemplate our use of a thin, highly adherent, low elongation coating to increase burst strength, and the scope of the present invention does not include soft buffer coatings for insulating stronger outer coatings on flashlamps.

What we claim is:

l. A photoflash lamp comprising, an hermetically sealed glass envelope, a combustion-supporting gas in said envelope, a quantity of combustible material lo cated in said envelope, ignition means attachedto said envelope and disposed in operative relationship to said combustible material, a first polymeric coating on the exterior surface of said glass envelope, said first coating being a polymeric material selected from the group consisting of cured bifunctional and polyfunctional epoxy resins of less than about percent elongation which are highly adherent to said glass envelope and thermosetting acrylic systems of less than about 5 percent elongation which are highly adherent to said glass envelope, and a second polymeric coating covering said first coating, said second coating having a percentage elongation of greater than about 5 percent.

2. A lamp according to claim 1 wherein said second coating comprises cellulose acetate lacquer.

3. A lamp according to claim 1 wherein the thickness of said first lamp coating is less than about 0.002 inch.

4. A lamp according to claim 1 wherein the thickness of said first coating is less than about 0.001 inch.

5. A lamp according to claim 2 wherein the thickness of said second coating is greater than about 0.005 inch.

6. A lamp according to claim 4 wherein the thickness of said second coating is within the range of about 0.005 to 0.015 inch.

7. A lamp according to claim 1 wherein the composition of said first coating is a polymeric material selected from the group consisting of cured bifunctional and polyfunctional epoxy resins of less than 5 percent elongation which are highly adherent to said glass, and said first coating is less than about 0.002 inch thick.

7 8. A lamp according to claim 7 wherein said second coating comprises cellulose acetate lacquer and has a thickness of from 0.005 to 0.015 inch.

9. A lamp according to claim 1 wherein said first coating comprises a bifunctional or polyfunctional epoxy resin cured with an amine or acid anhydride curing agent and having a percentage elongation of less than about 5 percent, said first coating has a thickness of less than about 0.001 inch, and said second coating comprises cellulose acetate lacquer and has a thickness of from about 0.005 to 0.015 inch.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5,852,125 Dated AUGUST 27, 1974 Inventor(s) THOMAS B. MC DONOUGH, JOHN W. SHAFFER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[75] In the Title:

Change: Thomas (P.) McDonough, Allenwood;

John W. Shaffer, Williamsport, both of Pa.

To: Thomas (-B.-) McDonough, Allenwood; John W. Shaffer, Williamsport, both of Pa.

Signed and sealed this 3rd day of December 1974.

(SEAL) Attest:

Mc'coY M. GIBSON JR. c. MARSHALL DANN Attesting Qfficer Commissioner of Patents l ORM PO-IOSO (10-59) USCOMM-DC 60376-P69 l 1* us sovznumzm- PRINTING OFFICE 1 I969 o-3es-:s4 

2. A lamp according to claim 1 wherein said second coating comprises cellulose acetate lacquer.
 3. A lamp according to claim 1 wherein the thickness of said first lamp coating is less than about 0.002 inch.
 4. A lamp according to claim 1 wherein the thickness of said first coating is less than about 0.001 inch.
 5. A lamp according to claim 2 wherein the thickness of said second coating is greater than about 0.005 inch.
 6. A lamp according to claim 4 wherein the thickness of said second coating is within the range of about 0.005 to 0.015 inch.
 7. A lamp according to claim 1 wherein the composition of said first coating is a polymeric material selected from the group consisting of cured bifunctional and polyfunctional epoxy resins of less than 5 percent elongation which are highly adherent to said glass, and said first coating is less than about 0.002 inch thick.
 8. A lamp according to claim 7 wherein said second coating comprises cellulose acetate lacquer and has a thickness of from 0.005 to 0.015 inch.
 9. A lamp according to claim 1 wherein said first coating comprises a bifunctional or polyfunctional epoxy resin cured with an amine or acid anhydride curing agent and having a percentage elongation of less than about 5 percent, said first coating has a thickness of less than about 0.001 inch, and said second coating comprises cellulose acetate lacquer and has a thickness of from about 0.005 to 0.015 inch. 